Previous work has shown that the enzymes responsible for the turnover of the phosphoryl moiety of nuclear phosphoproteins, the protein kinases and the phosphoprotein phosphatases, are present in 3-5 times higher concentration in the nuclei of rapidly growing tissues (hepatoma and fetal liver) compared to nuclei of adult liver (a non-growing tissue). In pursuance of these findings, the protein kinases and phosphoprotein phosphatases from the nuclei of hepatoma, adult and fetal livers will be purified to homogeneity. Their substrate specificity will be established and both catalysts and the substrates which they transform will be characterized and identified by means requiring small enough quantities of material so that changes in their activity and concentration in the cell can be monitored in cell culture, both in response to specific stimuli such as hormones, mitogens and serum factors and in relation to the cell cycle.